Carbon neutral hydrogen storage and release cycles based on dual-functional roles of formamides

The development of alternative clean energy carriers is a key challenge for our society. Carbon-based hydrogen storage materials are well-suited to undergo reversible (de)hydrogenation reactions and the development of catalysts for the individual process steps is crucial. In the current state, noble metal-based catalysts still dominate this field. Here, a system for partially reversible and carbon-neutral hydrogen storage and release is reported. It is based on the dual-functional roles of formamides and uses a small molecule Fe-pincer complex as the catalyst, showing good stability and reusability with high productivity. Starting from formamides, quantitative production of CO-free hydrogen is achieved at high selectivity ( > 99.9%). This system works at modest temperatures of 90 °C, which can be easily supplied by the waste heat from e.g., proton-exchange membrane fuel cells. Employing such system, we achieve >70% H2 evolution efficiency and >99% H2 selectivity in 10 charge-discharge cycles, avoiding undesired carbon emission between cycles.

Nonetheless, the concept presented here and ingenuity of finding another H2 carrier is an important discovery and in the future may lead to development of a more robust process.
Reviewer #2 (Remarks to the Author): In this manuscript Junge, Du, Beller and co-workers have reported the use of aqueous formamide as a hydrogen storage system. The high activity of iron pincer catalyst, release of highly pure H2 and the demonstration of 10 charge-discharge cycles are attractive features of this article. However, I am not convinced that the paper has novelty of the standard needed for the Nat Comm. The dehydrogenation process takes place via two steps: (i) hydrolysis of formamide under alkaline condition to form formate and amine and (ii) dehydrogenation of formate to CO2. Both of these steps have been independently well studied. For example, see Canadian Journal of Chemistry, Volume 80, Number 10, October 2002October , pp. 1343October -1350, and Angewandte Chemie International Edition, 45: 2893-2897. https://doi.org/10.1002/anie.200600283 for hydrolysis of formamides and J. Am. Chem. Soc. 2014, 136, 29, 10234-10237 for the dehydrogenation of formate using an iron-Macho pincer catalyst. The reverse reaction should be hydrogenation of CO2 in the presence of amines to make formamides. However, the reverse reaction mainly leads to the formation of formates as shown in Fig 4. In my opinion, this would not be classified as a reversible system as per the concept of LOHC. There are examples of earthabundant metal catalysts (manganese and iron) for the hydrogenation of CO2 and amines to formamides, for example see: ACS Catal. 2017, 7, 9, 6347-6351;ACS Catal. 2018, 8, 2, 1338-1345 Considering these precedences and concern with the reversibility of the process, I do not recommend the acceptance of this manuscript in Nat. Commun.
Reviewer #3 (Remarks to the Author): Authors report an elaborate description of reversible catalytic hydrogen storage medium which makes use of formamides as intermediates/bases. The results are novel with respect to the process as a whole -authors state that no single catalyst could perform all hydrogenation/dehydrogenation steps reported here in a stand-alone catalytic system and I concur.
Although some might note that catalysts involved in the study are not new, the process as a whole certainly is and i can highlight the extensive screening performed at the initial stage of the work which will help others in the field. I believe the work is of high scientific and technical quality and importance and suggest addressing the following questions: 1) Can authors clarify the origin of the ammine base effects they describe on page 10? It appears that pKa of the bases is not directly linked to the storage capacity of the system which is a little counterintuitive.
2) Iron pincer catalysts are well known to benefit from lewis acid promotion in formate dehydrogenation. Have authors utilized this in the case of formamides?
3) I believe that kinetic measurements are needed to demonstrate the catalyst stability implied in Figure  6. If the performance of the catalyst is indeed undegraded throughout the cycles then authors' claim is significantly more solid. 4) As with many iron pincers, the stability of the catalyst might be limited. Authors can provide pre-and postreaction 31P NMR to elaborate on the catalyst integrity throughout cycling experiment. This is also important for the peers working on deactivation mechanisms.

Reviewer #1 (Remarks to the Author):
It is an interesting concept and a very fundamental research. Data is concise and well analyzed.
The pitfall of the approach is in that the reactions are carried out in a mixture of very diluted organic solvent/H2O and in inert atmosphere of argon. Reported H2 purities (in high 90s%) aren't unusual if one works with millimolar concentrations The authors thank the reviewer for the comments. Actually, the catalytic dehydrogenation reaction couldn't take place under neat conditions or with H2O as sole solvent (see Supplementary Figure S10). Therefore, it's necessary to introduce organic solvent, preferably THF here, to increase the catalytic activity. The corresponding discussion is now revised as following: "Using water as sole solvent or under neat conditions, no hydrogen was found due to the low solubility of the catalyst." The presented 99%+ H2 purities using formamides as hydrogen storage materials is superior compared to the one using formic acid where equimolar CO2 is normally released together with H2. Besides, the hydrogen storage-release cycles could be scaled-up to 50 mmol while keeping the H2 purities at around 99.9%.
For the comment of "in inert atmosphere of argon", please see the response to the following remark.
Additionally, the fact that this is the first non-precious metal based system is undercut by the fact that the catalysts have to be stored in the dark and in the inert atmosphere. Preparation of extra dry and pure catalysts is more cost intensive than industrially-relevant precious metal catalysts. As such, applicability of this research would be limited to strictly research-based processes.
The authors thank the reviewer for the comments. Indeed, the iron pincer complexes are generally sensitive to oxygen, therefore the catalytic (de)hydrogenation reactions have to be set up in the inert atmosphere. However, once the H2 storage-release cycles are in operation, the whole system is closed and always under over-pressure of pure H2. On the other hand, air has also to be excluded from the system in order to suppress the hydrogen-air explosions (4.0 -75.6%v/v of H2 in air). Besides, the catalytic reactions utilize H2O/THF as co-solvent, therefore it's not necessary to prepare the system under extra dry conditions, thus simplifying the hydrogen storage process. Corresponding discussion is now added as following: "Even though the iron pincer complexes are generally sensitive to air (oxygen), once the H2 storagerelease cycles are in operation, the whole system is closed and generally under over-pressure of H2. On the other hand, air has also to be excluded from the system in order to suppress the hydrogen-air explosions (4. 0% -75.6%v/v of H2 in air)." CCU is an important and well known technology that is not constrained to organometallic community and one group, and as such, if the comparisons and callouts to this technology are to be made, authors need to make connections to currently utilized technologies and recent breakthroughs (other than academic literature, IEA website may be a good start for this topic).
The authors thank the reviewer for the comment. Corresponding discussion about currently applied CCU technologies is now added as following: "As one of the most prominent examples of CCU, the "George Olah Methanol Plant" in Iceland is based on local renewable energy and CO2. 26 Its total electrical energy demand and the overall efficiency reach 9.5 MWh/t methanol and 60%." Nonetheless, the concept presented here and ingenuity of finding another H2 carrier is an important discovery and in the future may lead to development of a more robust process.

Reviewer #2 (Remarks to the Author):
In this manuscript Junge, Du, Beller and co-workers have reported the use of aqueous formamide as a hydrogen storage system. The high activity of iron pincer catalyst, release of highly pure H2 and the demonstration of 10 charge-discharge cycles are attractive features of this article. However, I am not convinced that the paper has novelty of the standard needed for the Nat Comm. The dehydrogenation process takes place via two steps: (i) hydrolysis of formamide under alkaline condition to form formate and amine and (ii) dehydrogenation of formate to CO2. Both of these steps have been independently well studied. The authors thank the reviewer for the comments. We agree that the two steps: formamides hydrolysis and formates dehydrogenation are known in literature. However, what we exclusively show in the current study is that starting from formamides highly pure H2 (>99.9%) can be released during dehydrogenation reactions. This is in contrast to previous literature that reported application of formic acid or formates generally producing mixtures of CO2 and H2 in the dehydrogenation processes. Therefore, the presented hydrogen storage method might contribute to make hydrogen a relatively cleaner and more efficient energy carrier. Corresponding discussion is now added as following: "Even though the individual steps of formamides hydrolysis, FA (or formates) dehydrogenation and their reverse reactions are known, the presented hydrogen storage-release concept enables the reuse of in situ captured CO2, which allows to 1) retain the hydrogen storage material CO2 in the reaction, therefore, maintain the theoretical hydrogen storage capacity in successive H2 storage-release cycles, 2) avoid undesired carbon release during dehydrogenation processes, and 3) provide superior H2 selectivity/purity compared to other H2 carrier systems." The above-mentioned publications have been cited as references 65, 75, and 76 in the manuscript.
The reverse reaction should be hydrogenation of CO2 in the presence of amines to make formamides. However, the reverse reaction mainly leads to the formation of formates as shown in Fig 4. In my opinion, this would not be classified as a reversible system as per the concept of LOHC. There are examples of earth-abundant metal catalysts (manganese and iron) for the hydrogenation of CO2 and amines to formamides, for example see: ACS Catal. 2017, 7, 9, 6347-6351;ACS Catal. 2018, 8, 2, 1338-1345 The authors thank the reviewer for raising this point and fully agree that the hydrogenation of CO2 and amines to formamides is known in literature. In manuscript Figure 4, we show that the hydrogenation of CO2 or bicarbonate in the presence of amines leads to quantitative total yields of formates and formamides, which means that CO2 and bicarbonate can quantitatively store H2, preferably in the form of formates than formamides under relatively mild reaction conditions (90 °C, 12 h). On the other hand, we interpreted that the yields of formamides could be improved under harsher conditions (140 °C, 72 h, Supplementary Figure S3). Nevertheless, the milder conditions were adopted in catalytic reactions rather than the harsher ones, so H2 can be stored and released more efficiently and reversibly in the hydrogenation-dehydrogenation cycles, although the selectivity towards formamides is lower than that of formates. Corresponding discussion is added as following: "Although formate generation dominates at milder conditions (90 °C, 12 h), formamide yields could be improved at higher temperature and longer reaction time (140 °C, 72 h; Fig. S3), therewith formally clothing the formamide-based hydrogen storage cycle. However, due to practicability milder conditions were employed in subsequent catalytic (de)hydrogenation reactions, as this also allows for efficient and reversible H2 storage (Figs. 5b-c)." The above-mentioned publications have been cited as references 27 and 80 in the manuscript.
Considering these precedences and concern with the reversibility of the process, I do not recommend the acceptance of this manuscript in Nat. Commun.

Reviewer #3 (Remarks to the Author):
Authors report an elaborate description of reversible catalytic hydrogen storage medium which makes use of formamides as intermediates/bases. The results are novel with respect to the process as a whole -authors state that no single catalyst could perform all hydrogenation/dehydrogenation steps reported here in a stand-alone catalytic system and I concur.
Although some might note that catalysts involved in the study are not new, the process as a whole certainly is and i can highlight the extensive screening performed at the initial stage of the work which will help others in the field. I believe the work is of high scientific and technical quality and importance and suggest addressing the following questions: 1) Can authors clarify the origin of the ammine base effects they describe on page 10? It appears that pKa of the bases is not directly linked to the storage capacity of the system which is a little counterintuitive.
The authors thank the reviewer for raising this question. Unlike the normal acid-base reactions, the catalytic (de)hydrogenation reactions usually involve: substrates coordination, chemical bonds activation/splitting and products dissociation. Therefore, the amine base effects here are not just depending on their pKa but also other factors/properties like solubility, boiling point, hydrogen bonding, steric hindrance, and poisoning effect to the catalyst and so on. This could not be clarified so far. To make it easier for the audience, corresponding discussion including pKa values of amines are added in the main text (Figure 5a) as following: "As there is no obvious direct correlation of pKa of the applied amine and the storage capacity there will be other factors that potentially influence the system, i.e., solubility and boiling point of amines, hydrogen bonding, steric hindrance, catalyst poisoning etc." 2) Iron pincer catalysts are well known to benefit from lewis acid promotion in formate dehydrogenation. Have authors utilized this in the case of formamides?
The authors thank the reviewer for raising this question. We have performed the corresponding experiment using LiBF4 (10 mol%) as Lewis acid, however inferior H2 yield (85%) and selectivity (92.5%) were observed compared to the optimized conditions. Corresponding result is now added in the Supplementary Figure S10 and interpreted in the manuscript as following: "Lewis acids are known to assist dehydrogenation processes catalyzed by iron pincer catalysts. 65 However, inferior H2 yield (85%) and selectivity (92.5%) were observed in the presence of 10 mol% LiBF4 compared to the standard conditions (Figs. S10 and S34)." 3) I believe that kinetic measurements are needed to demonstrate the catalyst stability implied in Figure 6. If the performance of the catalyst is indeed undegraded throughout the cycles then authors' claim is significantly more solid.
The authors thank the reviewer's suggestion. The above-mentioned experiments have been carried out and corresponding results are summarized in Supplementary Table S1 and discussed in the manuscript as following: "Further, time dependent product generation of hydrogen storage and release reactions catalyzed by Fe-1 was investigated (Table S1). Lower total yields of formates and formamides were obtained in 3 and 6 h (66% and 87%, respectively) in hydrogenation reactions with morpholine (A1) and CO2. On the other hand, performing the dehydrogenation reactions with N-formylmorpholine (F1) in shorter reaction times led to decreased H2 yields (29% in 4 h and 49% in 8 h). These results demonstrate that long reaction times are indeed required." 4) As with many iron pincers, the stability of the catalyst might be limited. Authors can provide preand postreaction 31P NMR to elaborate on the catalyst integrity throughout cycling experiment. This is also important for the peers working on deactivation mechanisms.
The authors fully agree with reviewer's comment. The mentioned experiments have been performed. Indeed, a different signal in 31 P NMR spectra was observed at around 114 ppm after the catalytic dehydrogenation reaction (see Supplementary Figure S64), and is assigned to iron pincer derivative I-2 (see manuscript Figure 2 and J. Am. Chem. Soc. 2014, 136, 10234), which is considered as the resting state in (de)hydrogenation reactions. Corresponding discussion including the stability of iron pincer complexes is added as following: " 31 P NMR spectra of pre-and post-reaction samples (after 1 cycle) revealed that the original signal of Fe-1 complex (99.6 ppm) was shifted to lower field (114.0 ppm) after the catalytic dehydrogenation reaction (Fig. S64). This signal is assigned to iron pincer derivative I-2 (Fig. 2) and considered as the resting state in (de)hydrogenation reactions. 65 Besides, only minor species were found in the spectra which might either be the stereoisomers (e.g., trans-and cis-configurations) of the iron pincer complexes or their decomposition products. 87 "